One important parameter of an OLED device is the outcoupling efficiency, which is the fraction of the internally emitted photons that actually escape outside. Although conventional bottom LED engineering is rather mature, in general up to 80% of generated light is still unavailable for viewing, due to the losses in waveguiding modes, in the glass, ITO and organic layers and in surface plasmon modes in the metallic cathode.
One method that potentially can enhance the light outcoupling efficiency of an OLED device involves a microcavity structure, in which the organic EL elements are sandwiched between two reflecting mirrors, one of which is semitransparent, to let light escape. In most reported cases, a Quarter Wave Stack (QWS) is used as the semi-transparent reflector. A QWS, however, is a complicated structure and expensive to fabricate. In addition, a separated transparent electrode layer, usually an Indium Tin Oxide (ITO), is layered between the QWS and the organic layers, which further complicates the structure.
For active matrix OLED displays, a top emitting structure is preferred because the aperture ratio of the entire device can be significantly increased in this structure compared with that of the bottom emitting OLED structure. In a top emitting OLED, a highly reflective metal reflector is used in the anode structure to reflect the light back toward the cathode. In most reported cases, the reflecting metals have a high work function or are covered with a layer of transparent conductive oxide, e.g., ITO, which acts as carrier injection electrode.
U.S. Pat. No. 5,780,174 A1 (Tokito) provides a micro-optical resonator type organic electroluminescent device utilizing an organic electroluminescent material high in emission efficiency but broad in emission spectrum width, which a micro-optical resonator is formed by a multi-layered mirror and a metal mirror.
U.S. Pat. No. 5,949,187 (Xu) relates to an OED with a first microcavity including a first transparent spacer positioned adjacent the diode light output and a first mirror stack positioned on the first spacer to reflect light back into the OED and to define an optical length of the first microcavity. The optical length of the first microcavity emits light of a first spectrum. A second microcavity including a second transparent spacer is positioned adjacent the first microcavity and a second mirror stack positioned on the second spacer to reflect light toward the first microcavity and to define an optical length of the second microcavity.
U.S. Pat. No. 5,969,475 (Friend) discusses a light-emitting cavity device comprising a pair of mirrors spaced apart to define a resonant cavity; a luminescent layer located in the cavity and a control layer located in the cavity and controllable to adjust the resonance wavelength of the cavity and thereby spectrally redistribute the energy emitted by the luminescent layer.
U.S. Pat. No. 6,326,224 (Xu) relates to a method of purifying a primary color including providing an organic light emitting diode having a diode light output with a broad spectrum that includes a fraction of the primary color. The device includes a microcavity structure formed in cooperation with the organic light emitting diode to define an optical length of the microcavity structure, such that light emitted from the microcavity structure is the purified primary color.
U.S. Pat. No. 6,406,801 B1 (Tokito) discusses an optical resonator type organic electroluminescent element with a multilayered film mirror, a transparent electrode, an electron hole transportation layer, a luminescent layer configuring an organic layer, and a metallic electrode mirror formed on a glass substrate, which amplifies a specific wavelength (especially, in a range of about 30 nm toward a shorter wavelength side from a luminescence peak wavelength of the organic layer) in luminescence light by a minute optical resonator, which comprises the multilayered film mirror and the metallic electrode mirror.
U.S. Pat. No. 6,621,840 (Araki) is directed to a micro-optical resonator type organic light-emitting device comprising a substrate, a multi-layered thin film, a transparent electrode, an organic light-emitting layer, and a back electrode disposed on the substrate. If 0-mode resonance wavelength of the organic light-emitting device is expressed by λ0, n-mode resonance wavelength of the organic light-emitting device is expressed by λn (in which n is a positive integer) and reflectivity of the multi-layered thin film against light having a wavelength λ is expressed by Rλ, Rλ0 is 40% or more and Rλ1 or Rλ2 is 30% or less.
U.S. Pat. No. 6,639,250 (Shimoda) discusses a multiple-wavelength light emitting device comprising a light emitting layer for emitting light containing wavelength components to be output, a negative electrode that is positioned at the back surface of the light emitting layer and that transmits at least a portion of the light, reflecting layers positioned at the back surface of the negative electrode, for reflecting light having specific wavelengths, which reflecting layers are stacked in order perpendicularly to the light axis, in correspondence with the wavelengths of the light to be reflected, thus configuring a reflecting layer group. Divisions are made, in the direction perpendicular to the light axis, in any of at least two or more light emission regions that reflect light of different wavelengths. In each light emission region, the distance between the reflecting surface of the reflecting layer on the semi-transparent side and the reflecting surface in the semi-reflecting layer 2 is adjusted so that it becomes a resonating optical path length for the light that is emitted in the light emission region.
U.S. Pat. No. 6,812,637 (Cok) provides a top emitting OLED display, which includes a substrate; a patterned electrode formed above the substrate, defining a plurality of light emitting elements having gaps between the light emitting elements; a layer of OLED material disposed above the patterned electrode; a continuous transparent electrode disposed above the layer of OLED material; and a light-absorbing auxiliary electrode that is thermally and electrically conductive and in electrical and thermal contact with the continuous transparent electrode and located over the gaps between the light emitting elements of the display.
U.S. Pat. No. 6,861,800 (Tyan) discloses a color organic light-emitting display device having an array of pixels divided into at least two different color pixel sets, each color pixel set emitting a different predetermined color light over a common substrate, wherein each pixel in the array includes a metallic bottom-electrode layer disposed over the substrate and a metallic electrode layer spaced from the metallic bottom-electrode layer.
Notwithstanding significant improvements in LED and OLED technology, there exists a need for improved efficiency in light transmissions or outcoupling efficiency.